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Ultrafast Differential Sampling of Waveforms on Coplanar Transmission Lines

IP.com Disclosure Number: IPCOM000038567D
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 3 page(s) / 28K

Publishing Venue

IBM

Related People

Duling, IN: AUTHOR [+5]

Abstract

It has been demonstrated that an ultrashort electrical pulse can be generated on a coplanar transmission line by photoconductively shorting the charged line with a picosecond laser pulse [*]. The present publication describes an extension of this technique, known as the "sliding contact" technique, to the sampling of a waveform on a coplanar transmission line. The key advantage of this new configuration is that it provides a true differential sampling between the two lines forming the transmission line, at the point where it is being sampled. This is in sharp contrast with the traditional sampling which uses a side gap shorted by a laser pulse to recover the waveform on the transmission line, and mainly renders information about the signal on the line closest to the probe.

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Ultrafast Differential Sampling of Waveforms on Coplanar Transmission Lines

It has been demonstrated that an ultrashort electrical pulse can be generated on a coplanar transmission line by photoconductively shorting the charged line with a picosecond laser pulse [*]. The present publication describes an extension of this technique, known as the "sliding contact" technique, to the sampling of a waveform on a coplanar transmission line. The key advantage of this new configuration is that it provides a true differential sampling between the two lines forming the transmission line, at the point where it is being sampled. This is in sharp contrast with the traditional sampling which uses a side gap shorted by a laser pulse to recover the waveform on the transmission line, and mainly renders information about the signal on the line closest to the probe. The lay-outs for these two techniques are shown in the drawing. The unique implementation of this technique is described relative to a single coplanar transmission line in the case where the waveform is both generated and sampled by shorting the two lines with two laser beams at two distinct points. In addition, several situations are discussed, particularly in measurements of cross-talk between two such transmission lines, where the two techniques will yield different results which can only be explained by the true differential aspect of the method described here. The "sliding contact" sampling itself consists in photoconductively shorting with a short laser pulse, the two lines forming the transmission line.

The signal recovered at the end of the line by a slow current amplifier is then proportional to the instantaneous voltage difference between the two lines at the time when the laser pulse strikes. If the waveform is well synchronized with the sampling window that the laser pulse thus creates, it is possible to sample the instantaneous voltage at a given time in the waveform. By suitably delaying this sampling window with respect to the waveform, and repeating the same operation, the entire waveform can be recovered with a time resolution determined by the width of the sampling window, which can be subpicosecond, as shown in [*]. In the case where the line is initially charged, a synchronous detection scheme must be employed to isolate the sampled signal. For this, both the waveform to be sampled and the sampling window are amplitude modulated at two distinct low frequencies and the detecting...